Ultrafast Electronic Relaxation Dynamics in PbI2 Semiconductor Colloidal Nanoparticles: A Femtosecond Transient Absorption Study

Synthesis and surface engineering of Ag chalcogenide quantum dots for near-infrared biophotonic applications

Quantum dots (QDs), a novel category of semiconductor materials, exhibit extraordinary capabilities in tuning optical characteristics. Their emergence in biophotonics has been noteworthy, particularly in bio-imaging, biosensing, and theranostics applications. Although conventional QDs such as PbS, CdSe, CdS, and HgTe have garnered attention for their promising features, the presence of heavy metals in these QDs poses significant challenges for biological use. To address these concerns, the development of Ag chalcogenide QDs has gained prominence owing to their near-infrared emission and exceptionally low toxicity, rendering them suitable for biological applications. This review explores recent advancements in Ag chalcogenide QDs, focusing on their synthesis methodologies, surface chemistry modifications, and wide-ranging applications in biomedicine. Additionally, it identifies future directions in material science, highlighting the potential of these innovative QDs in revolutionizing the field.

Utilizing Machine Learning and Diode Physics to Investigate the Effects of Stoichiometry on Photovoltaic Performance in Sequentially Processed Perovskite Solar Cells

Organic-inorganic metal halide perovskite solar cells are renowned for their extensive solution processability, although the production of uniformly crystalline perovskite films can necessitate intricate deposition methods. In our study, we harmonized Shockley diode-based numerical analysis with machine learning techniques to extract the device characteristics of perovskite solar cells and optimize their photovoltaic performance in light of the experimental variables. The application of the Shockley diode equation facilitated the extraction of photovoltaic parameters and the prediction of power conversion efficiencies, thus aiding the understanding of device physics and charge recombination. Through machine learning, specifically Gaussian process regression, we trained models on current-voltage curves sensitive to variations in fabrication conditions, thereby pinpointing the optimal settings for enhanced device performance. Our multifaceted approach not only clarifies the interplay between experimental conditions and device performance but also streamlines the optimization process, diminishing the need for exhaustive trial-and-error experiments. This methodology holds substantial promise for advancing the development and fine-tuning of next-generation perovskite solar cells.

Edge Raman enhancement at layered PbI2platelets induced by laser waveguide effect

As a two-dimensional (2D) layered semiconductor, lead iodide (PbI₂) has been widely used in optoelectronics owing to its unique crystal structure and distinctive optical and electrical properties. A comprehensive understanding of its optical performance is essential for further application and progress. Here, we synthesized regularly shaped PbI₂platelets using the chemical vapor deposition method. Raman scattering spectroscopy of PbI₂platelets was predominantly enhanced when the laser radiated at the edge according to Raman mapping spectroscopy. Combining the outcome of polarized Raman scattering spectroscopy and finite-difference time domain simulation analysis, the Raman enhancement was proven to be the consequence of the enhancement effects inherent to the high refractive index contrast waveguide, which is naturally formed in well-defined PbI₂platelets. Because of the enlarged excited area determined by the increased propagation length of the laser in the PbI₂platelet formed waveguide, the total Raman enhancements are acquired rather than a localized point enhancement. Finally, the Raman enhancement factor is directly related to the thickness of the PbI₂platelet, which further confirms the waveguide-enhanced edge Raman. Our investigation of the optical properties of PbI₂platelets offers reference for potential 2D layered-related optoelectronic applications.

Few-Layer PbI2 Nanoparticle: A 2D Semiconductor with Lateral Quantum Confinement

Inspired by the superior optoelectronic performances of various 2D semiconductors, their new compositions and structures are being actively pursued in order to foster novel fundamental physics and device applications. As a layered semiconductor with a direct bandgap, few-layer PbI₂ should have drawn much research attention due to their capability of emitting photons at short wavelengths of the visible spectrum. Here we chemically synthesize few-layer PbI₂ flakes and nanoparticles, which demonstrate unique exciton properties that have rare counterparts in other 2D semiconductors. For three layers and more, the single PbI₂ flakes can be utilized to show how the bandgap energy of a 2D semiconductor evolves with the changing layer thickness. The single PbI₂ nanoparticles are associated with an ultranarrow photoluminescence line width of ∼1 meV, thus reflecting the influence of lateral quantum confinement on the energy-level structures of a 2D semiconductor. The above findings mark the emergence of a potent 2D platform that is more than complementary to well-studied transition-metal dichalcogenide monolayers.

Solution-Processed Lead Iodide for Ultrafast All-Optical Switching of Terahertz Photonic Devices

Solution-processed lead iodide (PbI₂ ) governs the charge transport characteristics in the hybrid metal halide perovskites. Besides being a precursor in enhancing the performance of perovskite solar cells, PbI₂ alone offers remarkable optical and ultrasensitive photoresponsive properties that remain largely unexplored. Here, the photophysics and the ultrafast carrier dynamics of the solution processed PbI₂ thin film is probed experimentally. A PbI₂ integrated metamaterial photonic device with switchable picosecond time response at extremely low photoexcitation fluences is demonstrated. Further, findings show strongly confined terahertz field induced tailoring of sensitivity and switching time of the metamaterial resonances for different thicknesses of PbI₂ thin film. The approach has two far reaching consequences: the first lead-iodide-based ultrafast photonic device and resonantly confined electromagnetic field tailored transient nonequilibrium dynamics of PbI₂ which could also be applied to a broad range of semiconductors for designing on-chip, ultrafast, all-optical switchable photonic devices.

Fabrication of CH₃NH₃PbI₃/PVP Composite Fibers via Electrospinning and Deposition

In our study, one-dimensional PbI₂/polyvinylpyrrolidone (PVP) composition fibers have been prepared by using PbI₂ and PVP as precursors dissolved in N,N-dimethylformamide via a electrospinning process. Dipping the fibers into CH₃NH₃I solution changed its color, indicating the formation of CH₃NH₃PbI₃, to obtain CH₃NH₃PbI₃/PVP composite fibers. The structure, morphology and composition of the all as-prepared fibers were characterized by using X-ray diffraction and scanning electron microscopy.

Whispering gallery mode lasing from hexagonal shaped layered lead iodide crystals

We report on the synthesis and optical gain properties of regularly shaped lead iodide (PbI2) platelets with thickness ranging from 10-500 nm synthesized by chemical vapor deposition methods. The as-prepared single crystalline platelets exhibit a near band edge emission of ∼ 500 nm. Whispering gallery mode (WGM) lasing from individual hexagonal shaped PbI2 platelets is demonstrated in the temperature-range of 77-210 K, where the lasing modes are supported by platelets as thin as 45 nm. The finite-difference time-domain simulation and the edge-length dependent threshold confirm the planar WGM lasing mechanism in such hexagonal shaped PbI2 platelet. Through a comprehensive study of power-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we ascribe the WGM lasing to be biexcitonic in nature. Moreover, for different thicknesses of platelet, the lowest lasing threshold occurs in platelets of ∼ 120 nm, which attributes to the formation of a good Fabry-Pérot resonance cavity in the vertical direction between the top and bottom platelet surfaces that enhances the reflection. Our present study demonstrates the feasibility of planar light sources based on layered semiconductor materials and that their thickness-dependent threshold characteristic is beneficial for the optimization of layered material based optoelectronic devices.

Exciton dynamics in semiconductor nanocrystals

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time-resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material-dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.

ROS-mediated apoptotic cell death in prostate cancer LNCaP cells induced by biosurfactant stabilized CdS quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) have raised great attention because of their superior optical properties and wide utilization in biological and biomedical studies. However, little is known about the cell death mechanisms of CdS QDs in human cancer cells. This study was designed to investigate the possible mechanisms of apoptosis induced by biosurfactant stabilized CdS QDs (denoted as "bsCdS QDs") in human prostate cancer LNCaP cells. It was also noteworthy that apoptosis correlated with reactive oxygen species (ROS) production, mitochondrial damage, oxidative stress and chromatin condensation in a dose- and time-dependent manner. Results also showed involvement of caspases, Bcl-2 family proteins, heat shock protein 70, and a cell-cycle checkpoint protein p53 in apoptosis induction by bsCdS QDs in LNCaP cells. Moreover, pro-apoptotic protein Bax was upregulated and the anti-apoptotic proteins, survivin and NF-κB were downregulated in bsCdS QDs exposed cells. Protection of N-acetyl cysteine (NAC) against ROS clearly suggested the implication of ROS in hyper-activation of apoptosis and cell death. It is encouraging to conclude that biologically stabilized CdS QDs bear the potential of its applications in biomedicine, such as tumor therapy specifically by inducing caspase-dependent apoptotic cell death of human prostate cancer LNCaP cells.

Investigation of ligand effects on exciton recombination in PbS nanoparticles

Multiple exciton generation (MEG) and exciton recombination were studied by femtosecond transient absorption spectroscopy in PbS nanoparticles (NPs) capped with oleic acid (PbS–OLA) and 2,3-dimercaptopropane sulfonate (PbS–DMPS) ligands. Analysis of the transient absorption data showed that the PbS–DMPS nanoparticles exhibit increased rates of multi- and single-exciton recombination compared with the PbS–OLA nanoparticles; however, the MEG yield for both sets of particles was the same within experimental error. The origin of the differences in the exciton recombination decay rates is unknown, but it is speculated that it may be due to the ionic functionality of DMPS or to the different ligating atoms of the OLA and DMPS ligands. The PbS–DMPS nanoparticles were highly sensitive to the presence of oxygen, which caused a dramatic increase in nonradiative decay pathways, which can be mistaken for multiexciton absorbance and decay. Removal of oxygen eliminated the nonradiative decay pathways. Overall, this study showed that the dynamics of MEG can be modified by changing the NP ligand shell, a result that may be useful in the development of NP-based thin-film solar devices.

Studies on optical absorption and photoluminescence of thioglycerol-stabilized CdS quantum dots

Nanoparticles of CdS were prepared at 303 K by aqueous precipitation method using CdSO4 and (NH4)2S in presence of the stabilizing agent thioglycerol. Adjustment of the thioglycerol (T) to ammonium sulphide (A) ratio (T:A) from 1:25 to 1:3.3 was done during synthesis and nanoparticles of different size were obtained. The prepared colloids were characterized by UV-vis and photoluminescence (PL) spectroscopic studies. Prominent first and second excitonic transitions are observed in the UV-vis spectrum of the colloid prepared with a T:A ratio of 1:3.3. Particle size analysis was done using XRD, high resolution TEM and dynamic light scattering and found to be approximately 3 nm. UV-vis and PL spectral features also agree with this particle size in colloid with T:A of 1:3.3. The band gap of CdS quantum dots has increased from the bulk value 2.4-2.9 eV. PL spectra show quantum size effect and the peak is shifted from 628 to 556 nm when the ratio of T:A was changed from 1:25 to 1:3.3. Doping of CdS with Zn2+ and Cu2+ is found to enhance the PL intensity. PL band shows blue-shift and red-shift on doping with Zn2+ and Cu2+, respectively. UV and PL spectral features of the CdS/Au hybrid nanoparticles obtained by a physical mixing of CdS and Au nanoclusters in various volume ratios is also discussed. Au red-shifts and rapidly quenches the PL of CdS. An additional low energy band approximately 650 nm is observed in the UV visible spectrum of the hybrid nanoparticles.